Among materials which may be employed as acoustic wave transmitting materials in SAW devices are materials such as lithium niobate and ST-quartz. ST-quartz has been found to be particularly suitable because it has a zero temperature coefficient of delay at room temperature for X-axis SAW propagation, thereby providing negligible delay changes due to variations in the temperature of the device. As a result, temperature control of a device constructed of ST-quartz is either unnecessary or a simple on-off heater has been found to be adequate. Unfortunately, however, problems arise when employing ST-quartz in a reflective array structure.
More specifically, in a reflective array structure such as a reflective array filter, the grating angle is determined in accordance with the ratio of two orthogonal SAW velocities. In ST-quartz, since the temperature coefficient of delay in a direction perpendicular to the X-direction is 47 ppm/.degree.C., the SAW velocities in two orthogonal directions change by different amounts approximately equal to 47 ppm/.degree.C. Therefore, the optimum grating angle undergoes a change as the operating temperature of the device is varied due to differential time delay changes and different linear expansions in the two respective orthogonal directions, design of the mask used to fabricate the ST-quartz device must incorporate a correction for this temperature differential effect, which necessitates a knowledge of fabrication and operating temperatures of the device beforehand. Thus, even though a conventional reflective array device constructed on ST-quartz may operate well at the temperature for which operation of the device was designed and will show little change in delay as the temperature is changed, the response of the device will fall off as the temperature deviates from the designed operating temperature. This roll-off as a function of temperature is depicted in FIG. 1, which is a graphic plot of signal loss versus temperature of a reflective array formed on an ST-quartz substrate, as reported in an article by P. C. Meyer and M. B. Schulz, entitled "Reflective SAW delay Line Material Parameters" in Ultrasonic Simposium Proceedings, page 500 (1973).
As is illustrated in FIG. 1 and reported in the above article, an ST-quartz grating suffers a 15 dB decrease in signal strength as the temperature is changed from -28.degree. C. to +42.degree. C. For some applications, this signal loss temperature profile is unacceptable, so that a conventionally configured grating must be modified or corrected in an effort to reduce the temperature sensitivity of the device.
Typically, the configuration of a reflective array, such as grating 1 or 2 as shown in FIG. 2, comprises a plurality of stripes 11, 12 configured as grooves or ribs formed in or on, respectively, a wave conducting material 15, such as ST-quartz. The pattern of the grooves or ribs by way of which the surface wave reflecting-discontinuities are created in the material is such that the opposite edges 11a, 11b, 12a, 12b of each rib or groove are parallel with each other and the separation 13 between opposing edges of adjacent grooves is constant along the grooves. As a result, for any direction of propogation of a surface wave, such as transmitted from transducer 18 along direction 14 and reflected by the array gratings 1 and 2 to be directed along direction 16 and received by transducer 19, the angle of incidence .theta. of the wave is the same for each stripe and at each point of incidence in each stripe. In other words, the grating angle is fixed at a constant value everywhere on the array, so that the array has a temperature sensitivity profile illustrated in FIG. 1.
Various schemes have been proposed to deal with the temperature sensitivity of SAW devices. For example, the U.S. Pat. No. 3,886,484 to Dias describes a substrate of differently located rotated Y-cuts cascaded together to expand the temperature range of the device by inclining plural faces of a device at respectively different angles. The U.S. Pat. No. 3,999,147 to Otto et al describes a device having reflective gratings with propogation directions in which the temperature coefficients of delay are opposite signs. Waves are reflected in two directions along path lengths to provide a summed linear temperature coefficient of delay. The U.S. Pat. Nos. 3,995,240 to Kerbel and 3,952,268 Schulz et al describe temperature-compensated devices in which an overlay of material deposited upon the surface structure is provided in an effort to compensate for temperature variations.